Draft (boiler)
The difference between atmospheric pressure and the pressure existing in the furnace or flue gas passage of a boiler is termed as draft. Draft can also be referred to the difference in pressure in the combustion chamber area which results in the motion of the flue gases and the air flow.
Types of Draft
Drafts are produced by the rising combustion gases in the stack, flue, or by mechanical means. For example, a blower can be put into four categories: natural, induced, balanced, and forced.
Natural draft: When air or flue gases flow due to the difference in density of the hot flue gases and cooler ambient gases. The difference in density creates a pressure differential that moves the hotter flue gases into the cooler surroundings.
Forced draft: When air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan.
Induced draft: When air or flue gases flow under the effect of a gradually decreasing pressure below atmospheric pressure. In this case, the system is said to operate under induced draft. The stacks (or chimneys) provide sufficient natural draft to meet the low draft loss needs. In order to meet higher pressure differentials, the stacks must simultaneously operate with draft fans.
